Amplifier



July 13, 1943. I M E 2,324,408

AMPLIFIER Filed Dec. :51, 1941 RECEIVING GIRCUI T AAAA ' //vv/v TOR" EGMARBLE/ qmk dt A T TORNE Y Patented July 13, 1943 UNITED STATES PATENT OFFICE AllIPLlFIER Frank G. Marble, East Orange, N..J., assignor to Western Electric Company, Incorporated, New York, N. Y., 'a corporation of New York Application December 31, 1941, Serial No. 425,091

4 Claims.

This invention relates to amplifiers and particularly to amplifiers involving feedback circuits.

In amplifiers for certain special applications such, for example, as sound level meters it is necessary to be able to change the gain by definite amounts. In such applications stable and uniform response over a wide range of frequencies is often required and this is conveniently obtained by using a large amount of negative feedback. In conventional circuits of this type, however, the gain cannot be changed within the feedback loop without at the same time afiecting the feedback circuit which in turn gives rise to a further rather indefinite change in gain which cannot be tolerated in applications of the type referred to above. While this could be avoided by adjusting the gain in some portion of the circuitnot within the feedback loop this usually involves the addition of other stages or circuit elements not otherwise required and in many'cases cost, weight or other considerations make it undesirable to resort to such expedients.

In accordance with this invention the gain of a feedback amplifier is made adjustable within the feedback loop by any given amount by adjusting the signal voltage effective at the grid of the tube to which the feedback is applied in such a manner that the impedance of the circuit to feedback currents is not changed. In the case of feedback to the grid of a tube, signal input voltages for the tube, differing in level by the amount required, are derived from suitable points on an impedance shunting the source of signals and the feedback circuit is connected to the impedance at an intermediate point such that the feedback potentials at the two points of connection to the grid are equal.

Fig l of the drawing shows an amplifying system according to the invention and Fig. 2 shows the equivalent of the bridge portion of the circuit of Fig. 1.

In this system a microphone l or other source of signal currents is connected by tandem vacuum tube amplifiers 2 and 3 to 'a receiving circuit or device 4. For purposes of illustration the amplifiers 2 and 3 are shown as single, self-biased battery operated triodes but obviously each amplifier may comprise any desired number of stages of any known type.

The output voltage of the amplifier 2 is impressed through condenser 5 on' the shunt impedance composed of the resistors 6, 1 and 8 in series and when the switch 9 is closed on the contact l0 a portion of the output voltage is effective at the grid ll of the amplifier 3. The voltage developed across resistor l2 in the output circuit of the amplifier 3 is applied through the coupling condenser l3 to the receiving circuit or device 4 where it is utilized for any desired purpose but a portion of this voltage is also fed back through the usual condenser I4 and resistor l5-to the junction of resistors 6 and I. From this point there are two paths to ground, one through resistors l and 8 and the other through condensers 5 and the output impedance of the amplifier 2. This latter impedance (R of Fig. 2) consists essentially of the plate resistance of the output tube of amplifier 2 and the resistance I6 in parallel 'since the impedances of condenser 5 and. the

power supply, which is shown in this case as a battery l1, will ordinarily be relatively very small for currents of signal frequencies.

When using a sound level meter to measure sounds of very low intensity, very high gain is required whereas less gain is sufiicient for ordinary purposes. Since the amplifier of such a meter is less likely to become unstable or to be affected by mechanical vibrations when in the less sensitive state, it is convenient to provide a switch for increasing the gain by 20 decibels above normal when low intensity sounds are to be measured. In the circuit shown, when the switch 9 is moved. from contact H) to contact l8, the gain is to be increased by this amount without affecting the feedback voltage efiective at the grid ll.

Since a change in level of 20 decibels is equiva lent to a voltage ratio of 10 itfollows that the signal voltage between point l9 and ground must be ten times the voltage between point 20 and ground. From simple bridge theory it follows that the feedback voltage at grid II will not be changed by the switching operation if the resistand the potential of point 20.is

where e=- .e and 8g is the signal voltage at the input of the amplifier 2 in each case.

For a 20-decibel change in gain 829 Rs 10 4 1OR =R5+R7+Rg s= a+ 1 But for the same feedback at either gain R (Equation 1 above) Substituting this value of Rs in Equation 6 9RR7= (Rs) +RR1 ('7) feedback path on the signal potentials of points l9. and 20 cannot be neglected the balance Equation 1 is unchanged but the following equation must be substituted for Equation 7 above T 7+ s+ 15 As is well known, the actual resistance of ordinary commercial resistors variesas much as 1-10 per cent of their nominal ratings and hence special resistors are ordinarily required where the design value is at all critical. It will be noted, however, that in this circuit relatively large deviations from the design value of resistors 1 and 8 are permissible since the changes in the loss produced by these variations are made negligibly small by the feedback action at the two settings of the switch 9. If, for example, the value of resistor I is too high then the potential drop across resistor 8 will be too low, the signal potential at point 20 will be more than 20 decibels below the potential of point I 9. However, the feedbackpotential at point 2|! will also be correspondingly too low and hence the net gain of the amplifier system tends to remain constant. Similarly other deviations from design value in the resistors I and 8 produce compensating changes in the amount of feedbac pplied to the amplifier 3.

While the invention has been described for purposes of illustration in connection with a sound level meter it is obviously applicable to any other system where a similar problem is involved and various changes may be made in the application and circuit details of the invention within the scope of the following claims.

7 What is claimed is:

1. In an amplifying system, first and second vacuum tubes connected in tandem, a negative feedback path around the second tube and means for changing the gain of the system without substantially affecting the feedback comprising a shunt impedance connected between.

put circuit to an intermediate point on the impedance and means for selectively connecting the input circuit to other points on the impedance at which equal potentials are produced by currents in the feedback path and unequal potentials are produced by currents from the signal source.

3. In an amplifying-system, a source of signal currents of predetermined impedance, three other impedances forming with the impedance of the source a balanced bridge, an amplifier having an input grid and an output circuit, a feedback path from the output circuit to diagonally opposite points of the bridge and means for selectively connecting the grid to the points on the bridge which are conjugate to the points of connection of the feedback circuit.

4. A system according to claim 3 in which the impedances forming the bridge are proportioned to produce achange of 20 decibels in the output level of the amplifier when the grid connection is moved from one of the conjugate points to the other.

FRANK G. MARBLE. 

